Quantifying exchangeable base cations in permafrost: a reserve of nutrients about to thaw

Permafrost ecosystems are limited in nutrients for vegetation development and constrain the biological activity to the active layer. Upon Arctic warming, permafrost thaw exposes large amounts of soil organic carbon (SOC) to decomposition and minerals to weathering but also releases organic and mineral soil material that may directly influence the soil exchange properties (cation exchange capacity, CEC, and base saturation, BS). The soil exchange properties are key for nutrient base cation supply (Ca2+, K+, Mg2+, and Na+) for vegetation growth and development. In this study, we investigate the distributions of soil exchange properties within Arctic tundra permafrost soils at Eight Mile Lake (Interior Alaska, USA) because they will dictate the potential reservoir of newly thawed nutrients and thereby influence soil biological activity and vegetation nutrient sources. Our results highlight much lower CEC density in surface horizons (similar to 9400 cmol(c) m(-3)) than in the mineral horizons of the active layer (similar to 16 000 cmol(c) m(-3)) or in permafrost soil horizons (similar to 12 000 cmol(c) m(-3)). Together, with the overall increase in CEC density with depth and the overall increase in BS (percentage of CEC occupied by exchangeable base cations Ca2+, K+, Mg2+, and Na+) with depth (from similar to 19 % in organic surface horizons to 62 % in permafrost soil horizons), the total exchangeable base cation density (Ca2+, K+, Mg2+, and Na+ in g m(-3)) is up to 5 times higher in the permafrost than in the active layer. More specifically, the exchangeable base cation density in the 20 cm upper part of permafrost about to thaw is similar to 850 g m(-3) for Ca-exch, 45 g m(-3) for K-exch, 200 g m(-3) for Mg-exch, and 150 g m(-3) for Na-exch. This estimate is needed for future ecosystem prediction models to provide constraints on the size of the reservoir in exchangeable nutrients (Ca, K, Mg, and Na) about to thaw.